1. Field of the Invention
The present invention relates generally to test switches/circuits used in conjunction with sensors which monitor the condition or status of important or critical functions in powered vehicles. In particular, the invention relates to a test switch/circuit which is used to indicate the status of the electrical conductors connecting to a remote sensor and is powered by the same conductors over which the sensor indication is provided. In a further embodiment the invention relates to a test switch/circuit used to verify the continuity of the conductor connection between a remote sensor and an alert indicator.
2. Background
In most powered vehicles such as automobiles, boats, and planes several sensors are used to monitor the status of various operating parameters to insure the safe and continued operation of the vehicle. Fuel, engine temperature, battery voltage, and oil pressure are typical operating parameters whose value or condition is presented to the vehicle operator. Sometimes an actual variable gauge is presented, while at other times warning lights are activated when the values for that parameter differ from established norms. The consequences of the failure of the monitoring systems are not uniform. A burned out bulb or broken wire in an automobile may leave a motorist stranded but not fatally injured. However, nowhere is the accuracy and dependability of such indicators more important than in flying aircraft.
In addition to sensors for powered performance, aircraft need to monitor the condition of the engine and transmission, lubrication and hydraulic systems, safety latches on hatches, etc. Typical sensors provide an open circuit between two conductors under normal conditions and close the circuit permitting current to flow when a fault condition is sensed. Typically, for a variety of reasons including cost it is desirable not to provide a separate power source and associated conductors for such sensors. Additionally, doing so introduces additional points for failure including the power source and the conductors leading to the sensors. Thus, it is most desirable to provide power to the sensors over the same electrical conductors as are used to convey the sensor signal. However, the sensors are generally located at some distance from the cockpit and are connected to the status indicators in the cockpit by conductors which may run over great lengths through several bulkheads, conduits, terminal blocks, and connectors. A connection failure anywhere along the path from sensor to cockpit indicator provides an open circuit. In such a case, the sensor may properly close the circuit upon detection of an appropriate condition but such information never reaches the cockpit through the open circuit. Failure of the warning system conductors could lead to catastrophic results since a fault condition would not be distinguishable from an open circuit condition.
For instance, engines and transmissions generally begin to wear internally before failure generating chips of metal which are picked up in the lubricating oil. Similarly, debris from both metallic wear and seal wear contaminate hydraulic systems. Thus, aircraft depend on sensors which detect contaminant particle accumulation in the lubricating or hydraulic systems. Many of these sensors use a magnet to draw metal particles out of the oil stream into contact with conductive electrodes on the face of the sensor. The sensor is normally placed in electrical series connection with a power source and an indicator lamp. One of the electrode contacts within the sensor is connected to ground either through a separate wire or by means of connection to the airplane chassis. The contacts within the sensor normally provide an open circuit so that no power is drawn through the indicator lamp. However, when sufficient metal particles have accumulated from the oil, the metal particles bridge the gap between the contacts to complete the circuit thereby permitting power to flow through the lamp to indicate to the pilots that trouble has developed. This type of normally open circuit is also known as a xe2x80x9cswitch to groundxe2x80x9d circuit and is employed in many aircraft sensors in addition to engine and transmission oil and hydraulic system sensors. Upon detection of the appropriate parameter, the sensors complete a circuit which draws power through an indicator lamp in the cockpit or otherwise activates circuits which indicate the fault.
To eliminate, or at least minimize, the chances that an open circuit would render a sensor signal inoperative, aircraft employ systems which check electrical continuity between critical sensors and the cockpit indicators each time the aircraft is powered up. Thus, when power is first switched on to the aircraft instruments before engine start-up, indicator lamps in the cockpit associated with critical functions are turned on for a interval of time sufficient for the pilots to notice the failure of any lamp to light. These lamps are typically turned on by test means which connect the power and ground conductors of the lamp circuit at or very near the associated sensor; that is, the test means provides an alternate current path (short) in parallel with the sensor which completes the series circuit. After a predetermined time, the test means ceases to short the circuit and the indicator lamp will go off unless the sensor itself completes the circuit.
As indicated above, generally it is not desirable to power such short circuit test means by use of additional wires (conductors) in an aircraft since adding such wiring to an aircraft is expensive and is itself subject to open circuit problems in the same manner as the sensor conductors. Thus, the short circuit test means are generally powered from the same conductor that supplies power (voltage) to the sensor. This requires that power be drawn from the sensor conductor for operation of the test means. One problem which is encountered is that drawing much power to power the test means from the sensor circuit during the test period reduces the voltage or current available to power the indicator lamp. A dim lamp is less easily detected under bright cockpit conditions. In other undisclosed embodiments of the invention, where additional functionality has been added, available power may be diminished to power the lamp.
3. Description of Related Art
One test means which is in common usage in aircraft is described by Berrier et al. in U.S. Pat. No. 5,045,840. Berrier teaches the use of a clock timer and sequencer in conjunction with a pulsing or intermittent switch which is placed in parallel with the sensor contacts. Closure of the switch across the leads shorts the sensor and permits current to flow through the power line turning on the lamp in the cockpit. The clock timer and sequencer control the duration of the test. Power is drawn from the sensor supply to power the clock timer and sequencer, but to avoid drawing too much power and dimming the lamp, the switch is not closed continuously but rather is opened and closed with a long duty cycle of approximately 90%. In the preferred embodiment the shorting switch is a MOS-FET transistor and is closed for 29 milliseconds each cycle and open for 1 millisecond each cycle. During the open time sufficient power is stored in a power supply capacitor to operate the circuit during the closed part of the cycle. Thus, in Berrier""s circuit the current through the lamp is pulsed, but at a rate imperceptible to the human eye. In addition, there is some diminution in the brightness of the indicator lamp since power is not being continuously provided to it. In addition, the continual opening and closing of the switch and abrupt power surges has the undesirable side effect of generating broadband electro-magnetic interference (EMI), high frequency signals associated with the Fourier transform of the voltage surge delta function each time the switch is opened or closed rapidly and repeatedly.
The invention described in this patent document provides a means for monitoring a piece of equipment such as an aircraft engine or transmission. More specifically, the present invention provides a means of testing the continuity of an electrical connection between an indicator and a remote sensor upon application of power to the circuit. The continuity test activates the indicator for a predetermined length of time if there is continuity of the conductor. After the completion of the desired test time, the test circuit no longer functions. The indicator lamp is not pulsed during the test time and full current is provided to the lamp generating its designed brightness. Further, the circuit across the sensor leads is not repetitively opened and closed and, therefore, no high frequency EMI is generated.
The switch/circuit of the invention works in conjunction with monitoring circuits typically found in aircraft. Among others, the cockpit monitoring circuit may consist of an alarm lamp in series with a direct current power supply, or a logic assembly, or it may be formed from an operational amplifier configured as a voltage comparator. When used with an alarm lamp, the current drawn through the sensor directly powers and illuminates the lamp. When used with a logic circuit or voltage comparator, a low voltage on the sensor line causes the monitoring circuit to light a panel lamp. In the case of a voltage comparator, the comparator compares a reference voltage at a first input to the voltage at a second input which depends on the status of the sensor. If the voltage at the second input is lower than the reference voltage, the comparator provides an output signal which can be used to power a warning lamp illumination circuit. If the voltage at the second input is equal to or higher than the reference voltage, no output signal is provided and the lamp illumination circuit is not activated.
In a circuit using an operational amplifier voltage comparator monitoring circuit, the second input is connected to one side of a normally open sensor and a series resistor to supply voltage. The voltage at the second input is determined by whether the sensor is open or closed. In the open position, the sensor does not connect the input to ground, and the voltage remains high. In the closed position, the sensor connects the second input to ground, driving down the voltage, which in turn causes the comparator to activate the lamp illumination circuit.
The test switch/circuit of this invention is also connected to the second input through the same conductor as the sensor. Upon power up, the oscillator part of the test switch/circuit is activated and appears as a short circuit (current path) to ground. Since the oscillator circuit serves the function of a switch by conducting current to ground, this patent document characterizes the invention by use of the term switch/circuit. If an indicator lamp is in series with the test circuit, current flows through the lamp and the test circuit to ground thereby illuminating the lamp. If a voltage comparator is used with the test circuit, the oscillator short to ground drives down the voltage at the second input and causes the comparator to activate the lamp illumination circuit. The test switch/circuit contains a timing function which turns off the oscillator circuit after a predetermined time so that it no longer appears as a short circuit (current path) to ground, voltage at the second input rises and the comparator no longer activates the lamp illumination circuit.
In a further embodiment, wherein the timing function is eliminated, the oscillator part of the circuit responsible for appearing as a short circuit (current path) to ground may also be used as an electronic switch in conjunction with a mechanical connection which moves the ferromagnetic core of a transformer in the oscillator circuit thereby decoupling the primary and secondary windings of the transformer.